Leading 30-27 with just moments to play, the visitors clung on superbly to finish on the opposite end of what was a mirror finish to last season's clash between the two sides, a game won by the Saints won at Donnybrook.
The Saints youngsters got a feel for the ball first but after conceding a penalty and then a free-kick, Leinster zipped the ball wide through the hands and Ian Fitzpatrick finished off the move.
The response that came from the hosts was superb and while Simon Uzokwe was dragged down short off the back of a rolling maul, advantage meant the Saints had another bite at the cherry. This time Jack Allcock remained bound and dropped to the ground over the whitewash.
The conversion went astray and Leinster's Conor McKeon, who had earlier slotted Leinster's extras extended the advantage with a penalty. The gap didn't last long though, Saints scrum-half Lewis Hall slicing Leinster apart and while what followed was a little scrappy, the ball was nevertheless kept alive for Josh Skelcey to cross in the corner.
As the pace of the game slowed down, Leinster recaptured the lead with a further McKeon penalty and the fly half maintained his 100 percent record from the tee by converting Stephen O'Neill's intercept on the half hour, also adding a drop goal for the half's last points.
The second half's first points, however, would go to the home side, Hall again proving to lively for Leinster as he sniped over from close range and Hooley's conversion was as simple as they come.
His next three points from a penalty were similarly routine and Leinster's half-time lead of 13 had been cut to just three in 10 second half minutes.
Again the game tightened up as the period went on and after wasting a good chip and chase through, Leinster full back Cian Kelleher made the most of a second bite of the cherry to again stretch the game away from the Saints.
The pattern was becoming increasingly familiar though and having slogged their way upfield, the Saints then earned a penalty try as their lineout maul was illegally hauled down.
Back to three again and the comeback was almost complete when Alex Wootton sped clear from his own 5m line. Leinster somehow scrambled to thwart the break and Wootton was denied minutes later, held up over the line as the clock ticked down.
Amazingly there was still time for two more Saints scrums but Leinster were not to be denied. The Saints pushed hard but a knock on ended proceedings and the Dubliners headed home with the glassware.
2mins LEIN Try Fitzpatrick Con McKeon 0-7
8mins SAINTS Try Allcock 5-7
12mins LEIN Pen McKeon 5-10
14mins SAINTS Try Skelcey 10-10
24mins LEIN Pen McKEON 10-13
29mins LEIN Try S O'Neill 10-20
34mins LEIN DG McKEON 10-23
HALF-TIME SAINTS 10 LEINSTER 23
40mins SAINTS Try Hall Con Hooley 17-23
45mins SAINTS Pen Hooley 20-23
50mins LEIN Try Kelleher Con McKeon 20-30
65mins SAINTS Try Penalty Con Mallinder 27-30
FULL-TIME SAINTS 27 LEINSTER 30